1. Field of the Invention
The present invention relates to an optical element used in an optical scanning apparatus and an optical scanning apparatus using such an optical element, and in particular, relates to an optical element suitable for use in an image forming apparatus such as a laser beam printer having an electrophotographic process, a digital copying machine, and a multifunction printer.
2. Description of the Related Art
Providing a diffraction grating on an entrance or exit surface of a curved refractive optical element to reduce chromatic aberration of a common optical system and to reduce degradation in optical performance due to environmental variations has been known. This method uses a physical phenomenon that chromatic aberration with respect to a light beam of a given reference wavelength appears in the opposite directions on a refracting surface and a diffracting surface of an optical system. Further, the fact that a great effect of reducing aberrations other than chromatic aberration of, for example, an aspherical lens can be achieved by changing the pitch of a periodic structure in a grating portion of the diffraction grating has been known (see Japanese Patent Application Laid-Open No. 2001-42112).
Particularly, an optical scanning apparatus used in an image forming apparatus such as a laser beam printer and digital copying machine guides a light flux emitted from a light source unit into a light deflector as a deflecting unit by an incident optical system. Then, an image forming optical system (fθ optical system) is caused to form an image of the light flux deflected and scanned by the light deflector on a photosensitive drum surface serving as a surface to be scanned like spots for optical scanning by the light flux on the photosensitive drum surface.
In such an optical scanning apparatus, a light flux emitted from the light source unit is converted into a parallel light flux by a collimator lens or the like and a linear image of the light flux converted into the parallel light flux to make a surface tilt error correction on a deflecting surface of a light deflector by a cylindrical lens. Then, the light flux deflected on the deflecting surface of the light deflector makes a constant-speed scan on the photosensitive drum surface by an image forming optical system to form spots.
Also in such an optical scanning apparatus, an incident optical system and an image forming optical system (image forming lens system) cause a light flux to converge or diverge by using a refractive optical element or a diffractive optical element (diffraction grating). If a diffractive optical element is used, an effect like an aspherical lens can be caused by changing the pitch of a periodic structure in the diffractive optical element so that an effect of significantly reducing aberrations and reducing degradation in optical performance due to environmental variations can be achieved.
However, while one light beam is also one light beam after diffraction on a refracting surface of a refractive optical element, light is divided depending on the diffraction order in diffraction on a diffracting surface of a diffractive optical element. Thus, if a diffractive optical element is used as an image forming optical system, it is necessary to set a specific order (design diffraction order) for the wavelength used. In addition, it is necessary to determine the shape of grating so that one light beam concentrated on the design diffraction order is generated. Further, if the shape of grating of the grating portion deviates from the desired shape, light beams of diffraction orders other than the design diffraction order are generated. This deviation from the desired shape of grating can lead to the generation of ghost light. Moreover, phase shifts with respect to a desired wave front (wave front aberrations) are caused and the peak quantity of light is reduced, which may prevent high-definition image formation. Therefore, it is necessary to carefully design and produce an image forming optical system using a diffractive optical element so that the shape of grating of the grating portion becomes the desired shape.
Various optical scanning apparatuses using a diffractive optical element have been proposed. See, for example, Japanese Patent Application Laid-Open No. 10-68903, Japanese Patent Application Laid-Open No. 2001-129714, and Japanese Patent Application Laid-Open No. 2007-293182.
In an optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 10-68903, a technology to optimize refractive power and diffraction power to correct changes in optical performance in the main scanning direction attendant on environmental variations is discussed. In an optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2001-129714, a technology to process an elliptical diffraction grating at high speed with high definition. In an optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2007-293182, a technology to reduce degradation in optical performance due to temperature changes by forming a stepped diffracting surface to be rotated in an incident optical system.
In a scanning optical system (optical scanning apparatus) having an elliptical diffraction grating whose diffracting surface is formed in a serrated shape, the grating (grating portion) (M0) on an optical axis is generally processed into a shape defined by a phase function, but slopes of other portions of the grating portion are formed linear due to restrictions of molding.
In an optical element formed based on general rotational symmetry as discussed in Japanese Patent Application Laid-Open No. 2001-42112, the shape of a grating portion as defined by a phase function is formed by bringing the tip of a cutting tool into contact with a work and continuously changing the cutting tool in the height direction while rotating the work.
Since it takes a long time to machine a diffraction grating in a conventional common diffractive optical element, an issue of lower accuracy of finishing is caused due to changes in ambient temperature and vibration of finishing machines.
However, if slopes of a grating portion are machined linear to reduce the machining time of a diffraction grating, a new issue arises that degradation in optical performance is caused due to shifts of a machined diffraction grating from a designed ideal diffraction grating.
In an optical system in which diffraction power of the diffraction grating (diffractive optical element) is weak, the grating pitch of a grating portion formed on the diffracting surface in a serrated shape becomes wider. Thus, if the method of linearly machining slopes of the grating portion is used, an error (machining error) deviating from the desired shape of the grating portion becomes greater. If an error of the shape of the grating portion of the diffraction grating becomes greater, a shift (wave front aberration) with respect to the wave front phase to be provided on the designed diffracting surface becomes larger, causing an issue that it becomes difficult to obtain desired optical characteristics due to lowering of a modulation transfer function (MTF) and degraded spots. Particularly when an elliptical diffraction grating is formed, it is difficult to machine the sectional shape of a diffracting surface (slope) as a curve, causing an issue of a greater error.